1. Field of the Invention
The present invention relates to hard-type high-structure carbon black which is capable of providing a rubber with a high modulus and excellent abrasion resistance and suitable for use in tire treads for various automobiles, and to a rubber composition comprising the carbon black.
2. Description of Background Art
Various kinds of carbon black possessing various respective characteristics have been used for rubber reinforcement. Performance of rubber compositions comprising carbon black is mainly determined by the characteristics of the carbon black. Therefore, carbon black having characteristics suitable for the purpose of the application has been selectively blended with rubbers.
In recent years, low fuel consumption tires have been developed to deal with demands for saving resources and energy. As such low fuel consumption tires, a rubber composition comprising a relatively small amount of carbon black with a large particle diameter which exhibits low heat build-up and high impact resilience is effective. However, although the blend of carbon black having a large particle diameter and a small specific surface area is effective for achieving low fuel consumption, a decrease in properties such as braking characteristics (handling stability) and abrasion resistance is unavoidable.
It is necessary to increase the hysteresis loss of the rubber composition in order to improve handling stability and provide high gripping performance. As a method of increasing the hysteresis loss of the rubber composition for tread, a method of filling a large amount of carbon black into a styrene-butadiene copolymer rubber (SBR) containing a large amount of bonded styrene has been employed. However, hardness of the rubber composition comprising a large amount of carbon black becomes so high that the frictional resistance to the road surface decreases. Moreover, such a rubber composition generates a remarkable amount of heat during travel, causing the rubber structure to deteriorate.
As described above, improvement of the gripping performance and abrasion resistance of tire tread is limited only by the amount of the rubber components and carbon black blended into the tire. Therefore, improvement of these rubber properties from the viewpoint of the characteristics of the carbon black has been attempted. Given as examples of such attempts are a radial tire for passenger cars comprising a rubber composition which comprises carbon black with a .DELTA.Dst/Dst of 0.7 or more for 100 parts by weight of a blended rubber of an SBR characterized in the amount of bonded styrene, amount of a vinyl bond at a butadiene site, and the like and other diene rubbers (Japanese Patent Application Laid-open No. 248441/1990); hard rubber black for achieving both superior gripping performance and excellent abrasion resistance, having a nitrogen adsorption specific surface area (N.sub.2 SA) of 120-165 m.sup.2 /g and a DBP absorption number of 120 ml/100 g or more in which a characteristic relation value (G value) for controlling the degree of surface activity and reducing the particle diameter is limited (Japanese Patent Application Laid-open No. 32137/1990); a rubber composition exhibiting superior abrasion resistance and excellent gripping performance with a small specific surface area and a small DBP absorption number, exhibiting superior abrasion resistance with a large specific surface area without either processability or abrasion resistance being impaired by mixing carbon black with a Dst of 70 or less, a nitrogen adsorption specific surface area/iodine adsorption number of 0.85-0.98, and a DBP absorption-24M4DBP of less than 20 (Japanese Patent Application Laid-open No. 174470/1991); and the like.
As a rubber composition exhibiting superior abrasion resistance and excellent gripping performance, Japanese Patent Application Laid-open No. 256577/1994 discloses a rubber composition comprising 50-180 parts by weight of carbon black having the following selective characteristics (1)-(5) for 100 parts by weight of diene rubber components.
(1) 140.ltoreq.CTAB.ltoreq.180, (2) -10.ltoreq.N.sub.2 SA-IA.ltoreq.0, PA0 (3) 110.ltoreq.24M4DBP.ltoreq.130, (4) 20.ltoreq.DBP-24M4DBP.ltoreq.40, PA0 (5) .DELTA.Dst.ltoreq.0.5Dst+23
As a method of providing high abrasion resistance in a rubber comprising carbon black, a method of increasing the structure of the carbon black has been considered to be effective. However, a sufficient increase in 24M4DBP which is an index of the structure level in the rubber cannot be achieved only by increasing the structure (increased DBP). Moreover, if the 24M4DBP value increases to a certain level, since the distribution of the aggregate tends to be broader as the 24M4DBP value increases, abrasion resistance is insufficient compared with the high 24M4DBP value.
Since abrasion resistance is generally improved in proportion to an increase in a modulus, improvement of abrasion resistance has resulted from the increase in a modulus to some extent. Rubber products are usually used by adjusting the modulus level to a predetermined level. Therefore, when the rubber is blended by being adjusted to a predetermined modulus level, improvement of abrasion resistance has been insufficient at such a modulus level.
Furthermore, a demand for low fuel consumption automobiles has become more and more important in view of environmental pollution. For example, reduction of the amount of carbon black to be included to provide lightweight tires, specifically, improvement of abrasion resistance by improving the characteristics of the carbon black has been demanded. Moreover, a higher modulus, superior abrasion resistance, and excellent gripping performance have been required for high performance tires accompanied by the development of high performance automobiles.